Observation of reduced turbulence scale length by electron cyclotron heating in KSTAR L-mode plasmas

Abstract

It is believed that anomalous transport of heat and particles is closely related to the micro turbulence which is caused by stiffness of the temperature or density profile. It is known that E x B flow shear, zonal flows and negative magnetic shear can suppress the micro turbulence. To understand these phenomena, a 2-D microwave imaging reflectometry (MIR) was installed on the KSTAR tokamak and has been improved to visualize electron density fluctuations in the 2-D (radial and poloidal) cross section [1]. In the 2014 KSTAR experiment, poloidal correlation lengths of density fluctuations which correspond to turbulent eddy structure sizes, were observed to decrease when electron cyclotron heating (ECH) was applied in L-mode plasmas. In the region of r/a < 0.7 (the ECH heating position is r/a ~ 0.6), the poloidal correlation length decreases over 10 % at f < 40 kHz however it remains constant at f > 40 kHz. The role of the ECH on the poloidal scale length reduction in the low frequency range is under investigation. Note that the radial correlation length measurement was unsuccessful because of the insufficient radial resolution. The MIR system will be upgraded to measure the radial correlation lengths by changing the electronics and increasing the number of radial detection channels. *Work supported by National Research Foundation of Korea under contract No. NRF-2014 M1A7A1A03029865 and US DoE under contract No. DE-FG-02-99ER54531.[1] W. Lee et al., Nuclear Fusion 54 (2014) 023012.